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Intrinsic Lead Ion Emissions in Zero-dimensional Cs4PbBr6 Nanocrystals Jun Yin, Yuhai Zhang, Annalisa Bruno, Cesare Soci, Osman M. Bakr, Jean-Luc Bredas, and Omar F. Mohammed ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01026 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017
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ACS Energy Letters
Intrinsic Lead Ion Emissions in Zero-dimensional Cs4PbBr6 Nanocrystals Jun Yin,1 Yuhai Zhang,1 Annalisa Bruno,2 Cesare Soci,2 Osman M. Bakr,1 Jean-Luc Brédas,3,* Omar F. Mohammed1,* 1
KAUST Solar Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia 2
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
3
School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
Corresponding Authors
[email protected];
[email protected].
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ABSTRACT We investigate the intrinsic lead ion (Pb2+) emissions in zero-dimensional (0D) perovskite nanocrystals (NCs) using a combination of experimental and theoretical approaches. The temperature-dependent photoluminescence experiments for both ‘non-emissive’ (highly suppressed green emission) and emissive (bright green emission) Cs4PbBr6 NCs show a splitting of emission spectra into high and low energy transitions in the UV spectral range. In the ‘nonemissive’ case, we attribute the high-energy UV emission at approximately 350 nm to the allowed optical transition of 3P1 to 1S0 in Pb2+ ions and the low-energy UV emission at approximately 400 nm to the charge transfer state involved in the 0D NC host lattice (D-state). In the emissive Cs4PbBr6 NCs, in addition to the broad UV emission, we demonstrate that energy transfer occurs from Pb2+ ions to green luminescent centers. The optical phonon modes in Cs4PbBr6 NCs can be assigned to both Pb-Br stretching and rocking motions from density functional theory (DFT) calculations. Our results address the origin of the dual broadband Pb2+ ion emissions observed in Cs4PbBr6 NCs and provide insights into the mechanism of ionic exciton-optical phonon interactions in these 0D perovskites.
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Metal-halide perovskites (MHPs) have attracted much attention over the past few years due to their promising applications in optoelectronic devices.1-8 In these MHPs, the basic building block is an octahedral BX6, where B is a metal and X is a halogen. The octahedral units can be arranged in different ways to form three-dimensional (3D), two-dimensional (2D), onedimensional (1D), and even zero-dimensional (0D) crystal structures.9-13 The 3D MHP14-17 structures with a general formula of ABX3 (A is an organic or inorganic cation with an extended network formed by the corner-sharing metal-halide octahedra) are the most extensively studied.18-20 In 2D structures, the octahedra are connected in layered or corrugated sheets separated by long organic cations.21-23 In 1D MHPs, the octahedral blocks assemble in a chain surrounded by organic cations.24 Finally, in the 0D cases, the octahedra are completely isolated from each other and surrounded by inorganic or organic cations; this leads to strong quantum confinement and strong exciton-phonon interactions.9 Specifically, the prototypical 0D building block, Cs4PbX6 (X = Cl, Br, or I)25 is expected to exhibit interesting photophysical properties because of the complete isolation of the [PbX6]4- octahedra that can cause exciton localization and self-trapping26 as well as small-polaron generation upon photoexcitation (unpublished results). The early work on Cs4PbX6 single crystals and thin films focused on their fundamental optical absorption and photoluminescence properties and attempted to distinguish their emission properties from those of ‘3D-like’ structures.27-33 These works have demonstrated that, unlike 3D and other low-dimensional perovskites, the main optical characteristics of Cs4PbX6 are determined by transitions between electronic states of the Pb2+ ions, and the observed broadband ultraviolet (UV) emission spectrum was ascribed to the radiative decay of a Frenkel exciton at
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Pb2+ sites.28 Thus, high-energy emissions from Pb2+ ions become the main optical feature for 0D Cs4PbX6. Recently, this basic understanding of the crystal structure and the optical properties of 0D Cs4PbX6 inspired many research groups to synthesize Cs4PbX6 nanocrystals (NCs) that are emissive in the visible (i.e., bright green luminescence) and ‘non-emissive’ (i.e., strongly suppressed or vanishing green luminescence).12, 34-38 For example, we reported green-emissive colloidal Cs4PbBr6 NCs using a reverse micro-emulsion method, which have high photoluminescence quantum yield.34,
39
However, Akkerman et al. have recently found that
Cs4PbBr6 NCs synthesized under Cs+-rich reaction conditions have no green emission and that they can be converted into green fluorescent CsPbBr3 NCs through a reaction with an excess of PbBr2.40 At the same time, Liu et al., have demonstrated the ligand-mediated transformation of presynthesized CsPbBr3 NCs to Cs4PbBr6 NCs via the addition of an amine.35 To clarify the green luminescent centers in emissive Cs4PbBr6 NCs, Quan et al., have proposed a model of lattice-matching between cubic CsPbBr3 NCs and a Cs4PbBr6 matrix.36 On the other hand, Bastiani et al. have provided new insights into the origin of the green emissions in Cs4PbBr6 single crystals by ruling out any role of CsPbBr3 nanocrystals or inclusion.12 Despite the progress made on the understanding of green emission from Cs4PbX6 NCs, the intrinsic emission features of Cs4PbX6 NCs as well as how the Pb2+ ion emissions affect the luminescent centers in the emissive NCs remain unclear. In this work, we study the intrinsic electronic behavior of both ‘non-emissive’ and emissive in the green Cs4PbBr6 NCs by combining temperature-dependent photoluminescence experiments and density functional theory (DFT) calculations. For ‘non-emissive’ Cs4PbBr6 NCs, our results show two broad UV emission spectra from Pb2+ ions with different origins. For emissive
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Cs4PbBr6 NCs, we show an energy transfer from Pb2+ ions to green luminescent centers, which can be significantly enhanced at high temperature due to the assistance of phonons. The relevant phonon modes of Cs4PbBr6 can be assigned via our DFT calculations to both Pb-Br rocking and stretching modes of each [PbBr6]4- octahedron.
Figure 1. (a) Temperature-dependent steady-state photoluminescence spectra of ‘non-emissive’ Cs4PbBr6 nanocrystals with the insert showing the TEM image; (b) temperature-dependent integrated PL intensity of high- and low-energy emission bands; (c) diagram of 3P1 to 1S0 and Dstate emissions from Pb2+ ions; projected density of states of 2×2×2 Cs4PbBr6 supercells calculated at the PBE level of theory (d) before and (e) after the replacement of a Cs atom with a Pb2+ ion (the optimized crystal structures of the supercells are also given).
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The ‘non-emissive’ and emissive Cs4PbBr6 NCs were prepared by controlling the addition amount of oleic acid and oleylamine ligands (see the Experimental Details). The X-Ray Diffraction (XRD) spectra of both samples are identical and confirm the zero-dimensionality of Cs4PbBr6 NCs (see Figure S1 of the Supporting Information). Both NCs have a uniform size of ~120 nm as confirmed by the transmission electron microscopy (TEM) images (the inserts in Figures 1a and 2a). The absorption spectra of both NCs can be found in Figure S2 of the Supporting Information, showing the main absorption peak around 320 nm. It is notable that the absorption onset for emissive NCs at 510 nm is completely suppressed in the “non-emissive” case. Figure 1a shows the normalized photoluminescence of ‘non-emissive’ NCs using a high excitation energy of 4.0 eV (above the bulk Cs4PbBr6 bandgap of ~3.9 eV). In addition to the significantly suppressed green emission at room temperature, the ‘non-emissive’ NCs show two strong broad emission bands in the UV spectral region with maxima at 348 nm (3.56 eV) and 404 nm (3.07 eV). The fitting of the high- and low-energy emission spectra as well as band maximum positions as a function of temperature can be found in Figures S3 and S4 (Supporting Information). As the temperature decreases, the total photoluminescence intensity of these two emission bands increases significantly due to the reduction of non-radiative recombination. While the high-energy emission band shifts continuously towards lower energy as the temperature decreases, the low-energy band shifts toward higher energy. The different shifting behavior suggests that these two emission bands come from different optical transitions related to Pb2+ ions that are occupying Cs+ sites. As illustrated in Figure 1c, the high-energy band is attributed to the allowed optical transition from the excited state (3P1) to the ground state (1S0) of the Pb2+ ion, which is consistent with previous studies on Pb2+ ion emissions in alkaline-earth coumpunds.41-42 The 3P1 excited state could be increasingly stabilized
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by the crystal field as the temperature decreases, leading to the red-shift of the high-energy band. On the other hand, the low-energy band can be assigned to the charge transfer state (or so-called D-state) emission of the Pb2+ ion in the host lattice41-42 and is due to the strong interaction between Pb2+ ions and [PbBr6]4- octahedra when Pb2+ ions occupy Cs+ sites in Cs4PbBr6 NCs. This interaction could be enhanced by the symmetry breaking or distortion of [PbBr6]4- octahedra due to Jahn-Teller effects (or s2 ion effects)43 in the ground state. To confirm the nature of the Dstate in the 0D host lattice, we performed density functional theory (DFT) calculations at the GGA/PBE level of theory on Cs4PbBr6 supercells before and after Pb2+ ion substitution. The optimized crystal structure and band structures along all symmetry k-points of bulk Cs4PbBr6 are shown in Figure S5 of the Supporting Information. Cs4PbBr6 shows a direct bandgap of 3.88 eV at the Γ point, which agrees well with the recently reported experimental bandgap of 3.95 eV29, 40 and the value of 3.99 eV calculated at the same level of theory.40 From the electronic projected density of states (PDOS) of a neutral supercell in Figure 1d, the top valence band is composed of both Br-3p and Pb-6p states and the bottom conduction band is dominated by the 6p states of Pb. Once a Cs atom is replaced by a Pb2+ ion from the Cs4PbBr6 crystal lattice (see Figure 1e), an intra-gap level (PDOS in red) from this Pb2+ ion (D-state) appears close to the conduction band edge. In this case, the excited electron could transfer from a [PbBr6]4- excited state level to the Dstate of a Pb2+ ion, which is achieved through the coupling between D-state and the vibrations of the octahedra in the 0D host lattice (a detailed analysis of the vibrational modes is given later). Therefore, higher temperatures lead to stronger D-state emission from Pb2+ ions located in a slightly distorted host environment, in relation to the strong coupling between the D-state and the Pb-Br bond vibrations in the [PbBr6]4- octahedra. At a low temperature, 77 K, the intensity of Dstate emission decreases in favor of the high-energy emission (3P1 → 1S0), due to the significant
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suppression of electron transfer from octahedra to D-states when the coupling becomes weak on the regular crystal lattice sites. Moreover, the D-state emission shifts continuously towards lower energy with increasing temperature due to lattice dilation (i.e., Pb2+ ions at Cs+ sites are in the dilated 0D host lattice), which follows the same trend as in typical semiconductors (like Si and GaAs).44 We further studied the nonradiative relaxation processes of Cs4PbBr6 NCs by fitting the integrated photoluminescence intensities of the two emission bands using the Arrhenius formula: 𝐼 = 𝐼0 /[1 + 𝑎𝑒𝑥𝑝(−𝐸𝑎 /𝑘𝑇)] , where Ea is the activation energy of the thermal quenching process, k is the Boltzmann constant, I0 is the zero-temperature PL intensity and a represents the strength of the quenching process. As shown in Figure 1b, the extracted activation energies are 37.8±1.9 meV for the high-energy band and 38.4±6.6 meV for the low-energy band; these are consistent with a previous report on Pb2+ ions in a Cs4PbBr6 thin film (40-43 meV)29. Since the presence of the charge compensating vacancy (vc-) strongly influences the optical characteristics of Pb2+ centers45, the activation energy for these emission bands could be ascribed to the localization energy of the exciton to Pb2+-vc- luminescent centers as no excess charge related to Pb2+ cations exists in the Cs4PbBr6 host lattice. After clarifying the nature of the high-energy luminescence states in ‘non-emissive’ Cs4PbBr6 NCs, we now turn to the photoluminescence properties of emissive Cs4PbBr6 NCs containing bright green luminescent centers (see Figure 2a). Similar to the ‘non-emissive’ Cs4PbBr6 NCs, at higher temperatures (180-300 K), the UV emission consists of two broad bands with maxima at approximately 340 and 400 nm (see the enlarged photoluminescence spectra in Figure S6 of the Supporting Information). The former can be still ascribed to the 3P1 → 1S0 transition on Pb2+ ions and the latter to the D-state emission. As the temperature decreases, the intensity of the latter
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decreases in favor of the former; once the temperature drops below 180 K, these two emission bands are indistinguishable due to the strong emission overlap. Since the sole broad UV emission band at low temperature is still composed of two emission bands, the proposed fast and slow emissive components in the previous study by Nikl et al.29 should be associated with the two Pb2+ ion emission bands we have described (vide supra).
Figure 2. (a) Temperature-dependent steady-state photoluminescence spectra of emissive Cs4PbBr6 NCs together with the TEM image in the inset; (b) diagrammatic sketch of UV and
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visible emission; (c, d) temperature-dependent integrated PL intensity of green emission using 310 and 470 nm excitations; and (e, f) full width at half maximum (FWHM) of green emission as a function of temperature using 310 and 470 nm excitations. The strong photoluminescence peak observed in the visible region is consistent with recent reports on emissive Cs4PbBr6 NCs,34,
36
where emission was either associated to ‘3D-like’
emitters imbedded in a 0D perovskite matrix,36 or ascribed to crystal structure defects (i.e., Br vacancy).12,
46
As shown in Figures 2c and 2d, the fitting Arrhenius plots of the integrated
photoluminescence intensity using 470 nm excitation gave an activation energy of 56.7±8.9 meV for the green emission (the spectra are shown in Figure S7 of the Supporting Information) and is comparable to the one obtained by 310 nm excitation (Ea = 52.3±1.6 meV). The activation energy for the green emission indicates that there is a thermally activated energy transfer process from Pb2+ ions and [PbBr6]4- octahedra to the green emission center upon high-energy excitation. We further analyzed the temperature-dependent emission broadening by extracting the full width at half-maximum (FWHM) of the PL spectra at different temperatures. This method has been used to study the mechanisms of electron-phonon coupling in perovskite materials.47 In these materials, different scattering mechanisms between charge carriers and phonons or impurities are associated with different functional dependencies of the PL linewidth Γ(T) on temperature, and can be expressed as the sum of several contributions48:
𝛤(𝑇) = 𝛤0 + 𝛾𝑎𝑐 𝑇 +
𝛾𝐿𝑂 𝐸𝐿𝑂
𝑒 𝑘𝑇 −1
𝐸𝑏
+ 𝛾𝑖𝑚𝑝 𝑒 −𝑘𝑇
where Γ0 is the temperature-independent inhomogeneous broadening term; 𝛾𝑎𝑐 𝑇 is the homogeneous broadening term which arises from acoustic phonon scattering through the
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deformation potential with charge carrier-acoustic phonon coupling strengths of γac; the third term is the homogeneous broadening term due to longitudinal optical (LO) phonon scattering with charge carrier-LO phonon coupling strengths of γLO; the last term is the scattering from ionized impurities with an average binding energy Eb. The acoustic phonons are expected to contribute significantly only in the low-temperature region (T
